Power converters find use in many fields for switching power to drive loads. Power converters may comprise networks of parallel and/or series connected power switching devices, particularly semiconductor switching devices such as IGBTs, FETs, MOSFETS and the like the switching of which is controlled by a gate drive.
Particularly for high power, high current applications, power switching devices are connected in parallel. This allows power losses to be distributed between several devices as well as to achieve high power within the size limitations of such switching devices. Distributing losses between several devices means that there is a lower risk of overheating at the device junction, thus improving the life of the switching devices.
A problem that has been identified with using paralleled switching devices is that if the devices are not identical (in almost all cases the devices will not be identical due to e.g. manufacturing and material tolerances), the current passing through the respective devices will also not be identical i.e. there may be a current imbalance. This may cause damage to the devices e.g. due to overheating. Even if the devices are nominally equivalent, there may very well be minor differences due to manufacturing tolerances, differences in physical dimensions, age, carrier concentration, materials etc. Alternatively, current imbalances can arise due to different device junction temperatures or gate drive imbalances. Damage can cause a switch to operate outside of its safe operating area.
Conventionally, this has been compensated by measuring the currents through the respective switching devices and compensate for the differences.
To compensate for such differences, it has been known to couple a passive component such as an inductor in series with the switching devices to reduce the imbalance in current sharing during transient—i.e. on switching on/power up and also adding in active control devices to counteract imbalances during steady state operation. If the switching devices are MOSFETs, these will also provide some self-balancing of current variation due to their positive temperature coefficient. Adding series inductance impedance in the power path, however, brings its own problems in that it slows down the switching and can generate a ringing effect with the parasitic element of the switching device. This adversely affects the EMI of the power converter.
It is desired to provide a power converter paralleled switching devices control method and arrangement which overcomes these problems, is non-intrusive, allows lower EMI and provides better control of current imbalance particularly during switching on, but without the need for a complex current system approach.